Last year’s capture of Mexican drug kingpin Joaquín “El Chapo” Guzmán led to a surge in homicides in Mexico as cartel leaders fought to fill the vacuum created by his arrest.

The apprehension of Guzmán in January 2016 was hailed by Mexican and U.S. officials as a watershed moment in the war on drugs. But Mexico's homicide rate for the year spiked to 21.3 murders per 100,000 inhabitants, a steep rise from 17.5 in 2015 that rivals record numbers earlier in the decade, according to a report released Friday by the Justice in Mexico Project at the University of San Diego.

Mexico had just started emerging from its bloody battle with drug cartels, with murder rates dropping for four consecutive years from 2011 to 2014. After the removal of Guzmán, violence is back on the rise. The drug lord was extradited to the United States in January to face criminal charges for his leadership of the trafficking syndicate known as the Sinaloa Cartel.

"It's kind of two steps forward, one step back," said David Shirk, director of the Justice in Mexico Project and co-author of the report. "We took out a very powerful and important drug trafficker. But as a result, we have destabilized the ecosystem of organized crime in a way that has led to internal struggles within the Sinaloa Cartel, and encroachment from other organizations that would like to take over their business."

The spike in violence also helps explain why the United States is seeing a resurgence in heroin use. The problem has become so widespread that President Trump created a national opioid addiction commission. On Wednesday he hosted a White House "listening session" with addicts, including one recovering heroin addict.

Shirk said the battles between Mexican drug cartels have upset the "traditional" drug routes — including cocaine — that originate in South America and funnel through Mexico to the U.S. That has made it more difficult for American users to find cocaine, opening the door for heroin and other opioids, which can be produced in Mexico and smuggled more easily into the U.S.

Heroin profits are smaller, Shirk said, but they provide those cartels with quick and easy cash as they focus on fighting for control of territories left vacant by Guzmán's arrest.